The goals of this program are to explore the applications of radioactive tracers in medical diagnosis and biomedical research, to develop these methods to the point of wide-spread application, evaluate them critically and use them in selected areas of our own research. Part of our program is concerned with the development of new instruments for visualizing and quantifying the distribution of radioactive tracers within the human body and with instruments designed for in vitro procedures. Radiochemicals and techniques are developed and then applied when possible to clinical investigation. By measuring the spatial and temporal distribution of radioactive tracers, we attempt to answer the question, "where" physiologic and biochemical processes are occurring within the body as well as "how fast." Frequently we are able to measure safely in man processes that in the past could only be studied in experimental animals. Such studies are possible with the use of shortlived radioactive tracers. The program has been directed in seven major directions, each supervised by a project leader. These areas of investigation are as follows: (1) structure and function of the reticuloendothelial system; (2) studies of the circulation; (3) radiopharmaceutical research and development; (4) automation of microbiology; (5) systems analysis in nuclear medicine; (6) computer applications in nuclear medicine; and (7) metabolic and kinetic studies.